Communicating with an Implanted Wireless Sensor

ABSTRACT

The present invention determines the resonant frequency of a sensor by adjusting the phase and frequency of an energizing signal until the frequency of the energizing signal matches the resonant frequency of the sensor. The system energizes the sensor with a low duty cycle, gated burst of RF energy having a predetermined frequency or set of frequencies and a predetermined amplitude. The energizing signal is coupled to the sensor via magnetic coupling and induces a current in the sensor which oscillates at the resonant frequency of the sensor. The system receives the ring down response of the sensor via magnetic coupling and determines the resonant frequency of the sensor, which is used to calculate the measured physical parameter. The system uses a pair of phase locked loops to adjust the phase and the frequency of the energizing signal.

RELATED APPLICATION

This application is a continuation patent application of U.S. patentapplication Ser. No. 11/105,294, entitled “Communication with anImplanted Wireless Sensor” filed Apr. 13, 2005 which claims priority toU.S. Provisional Application No. 60/623,959 entitled “Communicating withan Implanted Wireless Sensor” filed Nov. 1, 2004, all of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention is directed in general to communicating with awireless sensor, and in particular to communicating with a wirelesssensor implanted within the body to measure a physical condition.

BACKGROUND

Wireless sensors can be implanted within the body and used to monitorphysical conditions, such as pressure or temperature. For example, U.S.Pat. No. 6,111,520, U.S. Pat. No. 6,855,115 and U.S. Publication No.2003/0136417, each of which is incorporated herein by reference, alldescribe wireless sensors that can be implanted within the body. Thesesensors can be used to monitor physical conditions within the heart oran abdominal aneurysm. An abdominal aortic aneurysm (AAA) is adilatation and weakening of the abdominal aorta that can lead to aorticrupture and sudden death. In the case of a repaired abdominal aneurysm,a sensor can be used to monitor pressure within the aneurysm sac todetermine whether the intervention is leaking. The standard treatmentfor AAAs employs the use of stent-grafts that are implanted viaendovascular techniques. However, a significant problem that has emergedwith these stent-grafts for AAAs is acute and late leaks of blood intothe aneurysms sac. Currently, following stent-graft implantation,patients are subjected to periodic evaluation via abdominal CT (ComputedTomography) with IV contrast to identify the potential presence ofstent-graft leaks. This is an expensive, risky procedure that lacksappropriate sensitivity to detect small leaks.

Typically, the sensors utilize an inductive-capacitive (“LC”) resonantcircuit with a variable capacitor. The capacitance of the circuit varieswith the pressure of the environment in which the sensor is located andthus, the resonant frequency of the circuit varies as the pressurevaries. Thus, the resonant frequency of the circuit can be used tocalculate pressure.

Ideally, the resonant frequency is determined using a non-invasiveprocedure. Several examples of procedures for determining the resonantfrequency of an implanted sensor are discussed in U.S. Pat. No.6,111,520. Some of the procedures described in the patent require thetransmission of a signal having multiple frequencies. A drawback ofusing a transmission signal having multiple frequencies is that theenergy in the frequency bands outside the resonant frequency is wasted.This excess energy requires more power which results in an increase incost, size, and thermal requirements, as well as an increase inelectromagnetic interference with other signals. Thus, there is a needfor an optimized method that is more energy efficient and requires lesspower.

There are unique requirements for communicating with an implantedsensor. For example, the system must operate in a low power environmentand must be capable of handling a signal from the sensor with certaincharacteristics. For example, the signal from the sensor is relativelyweak and must be detected quickly because the signal dissipates quickly.These requirements also impact the way that common problems are handledby the system. For example, the problems of switching transients andfalse locking need to be handled in a manner that accommodates thesensor signal characteristics. Thus, there is a need for a method forcommunicating with a wireless sensor that operates in a low powerenvironment and that efficiently determines the resonant frequency ofthe sensor.

The resonant frequency of the sensor is a measured parameter that iscorrelated with the physical parameter of interest. To be clinicallyuseful there must be means to ensure that variations in measurementenvironment do not affect the accuracy of the sensor. Thus, there is aneed for a system and method for communicating with a wireless sensorthat considers variations in the measurement environment.

SUMMARY OF THE INVENTION

The primary goal of aneurysm treatment is to depressurize the sac and toprevent rupture. Endoleaks, whether occurring intraoperatively orpostoperatively, can allow the aneurysmal sac to remain pressurized andtherefore, increase the chance of aneurysm rupture. The current imagingmodalities angiography and CT scan are not always sensitive enough todetect endoleaks or stent graft failure. Intrasac pressure measurementsprovide a direct assessment of sac exclusion from circulation and maytherefore offer intraoperative and post operative surveillanceadvantages that indirect imaging studies do not.

In one application of the present invention, an AAA pressure sensor isplaced into the aneurysm sac at the time of stent-graft insertion. Thepressure readings are read out by the physician by holding an electronicinstrument, which allows an immediate assessment of the success of thestent-graft at time of the procedure and outpatient follow-up visits, byreading the resonant frequency of the wireless sensor and correlatingthe frequency reading to pressure.

The present invention meets the needs described above by providing asystem and method for communicating with a wireless sensor to determinethe resonant frequency of the sensor. The system energizes the sensorwith a low duty cycle, gated burst of RF energy having a predeterminedfrequency or set of frequencies and a predetermined amplitude. Theenergizing signal is coupled to the sensor via a magnetic loop. Thesensor may be an inductive-capacitive (“LC”) resonant circuit with avariable capacitor that is implanted within the body and used to measurephysical parameters, such as pressure or temperature. The energizingsignal induces a current in the sensor which is maximized when theenergizing frequency is the same as the resonant frequency of thesensor. The system receives the ring down response of the sensor viamagnetic coupling and determines the resonant frequency of the sensor,which is used to calculate the measured physical parameter.

In one aspect of the invention a pair of phase locked loops (“PLLs”) isused to adjust the phase and the frequency of the energizing signaluntil its frequency locks to the resonant frequency of the sensor. Inone embodiment, one PLL samples during the calibration cycle and theother PLL samples during the measurement cycle. These cycles alternateevery 10 microseconds synchronized with the pulse repetition period. Thecalibration cycle adjusts the phase of the energizing signal to a fixedreference phase to compensate for system delay or varying environmentalconditions. The environmental conditions that can affect the accuracy ofthe sensor reading include, but are not limited to, proximity ofreflecting or magnetically absorbative objects, variation of reflectingobjects located within transmission distance, variation of temperatureor humidity which can change parameters of internal components, andaging of internal components.

One of the PLLs is used to adjust the phase of the energizing signal andis referred to herein as the fast PLL. The other PLL is used to adjustthe frequency of the energizing signal and is referred to herein as theslow PLL. During the time that the energizing signal is active, aportion of the signal enters the receiver and is referred to herein as acalibration signal. The calibration signal is processed and sampled todetermine the phase difference between its phase and the phase of alocal oscillator (referred to herein as the local oscillator 2). Thecycle in which the calibration signal is sampled is referred to as thecalibration cycle. The system adjusts the phase of the energizing signalto drive the phase difference to zero or another reference phase.

During the measurement cycle, the signal coupled from the sensor(referred to herein as the coupled signal or the sensor signal) isprocessed and sampled to determine the phase difference between thecoupled signal and the energizing signal. The system then adjusts thefrequency of the energizing signal to drive the phase difference to zeroor other reference phase. Once the slow PLL is locked, the frequency ofthe energizing signal is deemed to match the resonant frequency of thesensor. The operation of the slow PLL is qualified based on signalstrength so that the slow PLL does not lock unless the strength of thecoupled signal meets a predetermined signal strength threshold.

The system also handles false locking and switching transients. A falselock occurs if the system locks on a frequency that does not correspondto the resonant frequency of the sensor. In one aspect of the invention,the system avoids false locks by examining how the phase differencesignal goes to zero. If the slope of the phase difference signalrelative to time meets a predetermined direction, e.g. positive, thenthe PLL is allowed to lock. However, if the slope of the phasedifference signal relative to time does not meet the predetermineddirection, e.g. it is negative, then the signal strength is suppressedto prevent a false lock.

Another aspect of the invention uses frequency dithering to avoid afalse lock. A constant pulse repetition frequency can add spectralcomponents to the sensor signal and cause a false lock. By randomlyvarying the pulse repetition frequency of the energizing signal, thesidebands move back and forth so that the average of the sidebands isreduced. Thus, the system locks on the center frequency rather than thesidebands.

In another aspect of the invention, phase dithering can be used toreduce switching transients. The phase of the energizing signal and alocal oscillator (referred to herein as local oscillator 1) are randomlychanged. Varying the phase of the energizing signal varies the phase ofthe coupled signal, but does not affect the phase of the transientsignal. Thus, the average of the transient signal is reduced. Changingthe resonant frequency of the coil as it is switched from energizingmode to coupling mode also reduces switching transients. The capacitorsthat are connected to the coil are switched between different modes toslightly change the resonant frequency in order to reduce switchingtransients.

These and other aspects, features and advantages of the presentinvention may be more clearly understood and appreciated from a reviewof the following detailed description of the disclosed embodiments andby reference to the appended drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary system for communicating witha wireless sensor in accordance with an embodiment of the invention.

FIG. 2(a) is a graph illustrating an exemplary energizing signal inaccordance with an embodiment of the invention.

FIGS. 2(b), 2(c) and 2(d) are graphs illustrating exemplary coupledsignals in accordance with an embodiment of the invention.

FIG. 3 is a block diagram of an exemplary base unit in accordance withan embodiment of the invention.

FIGS. 4(a) and 4(b) are graphs illustrating exemplary phase differencesignals in accordance with an embodiment of the invention.

FIG. 5 illustrates frequency dithering in accordance with an embodimentof the invention.

FIG. 6 illustrates phase dithering in accordance with an embodiment ofthe invention.

FIG. 7 illustrates a coupling loop in accordance with an embodiment ofthe invention.

FIG. 8 is a graph illustrating an exemplary charging response of an LCcircuit in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

The present invention is directed towards a system and method forcommunicating with a wireless sensor. Briefly described, the presentinvention determines the resonant frequency of the sensor by adjustingthe phase and frequency of an energizing signal until the frequency ofthis signal locks to the resonant frequency of the sensor. The systemenergizes the sensor with a low duty cycle, gated burst of RF energy ofa predetermined frequency or set of frequencies and predeterminedamplitude. This signal induces a current in the sensor that can be usedto track the resonant frequency of the sensor. The system receives thering down response of the sensor and determines the resonant frequencyof the sensor, which is used to calculate the measured physicalparameter. The system uses a pair of phase locked loops (“PLL”s) toadjust the phase and the frequency of the energizing signal to track theresonant frequency of the sensor.

Exemplary System

FIG. 1 illustrates an exemplary system for communicating with a wirelesssensor implanted within a body. The system includes a coupling loop 100,a base unit 102, a display device 104 and an input device 106, such as akeyboard.

The coupling loop is formed from a band of copper. In one embodiment,the loop is eight inches in diameter. The coupling loop includesswitching and filtering circuitry that is enclosed within a shielded box101. The loop charges the sensor and then couples signals from thesensor into the receiver. The antenna can be shielded to attenuatein-band noise and electromagnetic emissions.

Another possible embodiment for a coupling loop is shown in FIG. 7,which shows separate loops for energizing 702 and for receiving 704,although a single loop can be used for both functions. PIN diodeswitching inside the loop assembly is used to provide isolation betweenthe energizing phase and the receive phase by opening the RX path pindiodes during the energizing period, and opening the energizing path pindiodes during the coupling period. Multiple energizing loops can bestaggered tuned to achieve a wider bandwidth of matching between thetransmit coils and the transmit circuitry.

The base unit includes an RF amplifier, a receiver, and signalprocessing circuitry. Additional details of the circuitry are describedbelow in connection with FIG. 3.

The display 104 and the input device 106 are used in connection with theuser interface for the system. In the embodiment illustrated in FIG. 1the display device and the input device are connected to the base unit.In this embodiment, the base unit also provides conventional computingfunctions. In other embodiments, the base unit can be connected to aconventional computer, such as a laptop, via a communications link, suchas an RS-232 link. If a separate computer is used, then the displaydevice and the input devices associated with the computer can be used toprovide the user interface. In one embodiment, LABVIEW software is usedto provide the user interface, as well as to provide graphics, store andorganize data and perform calculations for calibration andnormalization. The user interface records and displays patient data andguides the user through surgical and follow-up procedures.

An optional printer 108 is connected to the base unit and can be used toprint out patient data or other types of information. As will beapparent to those skilled in the art other configurations of the system,as well as additional or fewer components can be utilized with theinvention.

Patient and system information can be stored within a removable datastorage unit, such as a portable USB storage device, floppy disk, smartcard, or any other similar device. The patient information can betransferred to the physician's personal computer for analysis, review,or storage. An optional network connection can be provided to automatestorage or data transfer. Once the data is retrieved from the system, acustom or third party source can be employed to assist the physicianwith data analysis or storage.

FIG. 1 illustrates the system communicating with a sensor 120 implantedin a patient. The system is used in two environments: 1) the operatingroom during implant and 2) the doctor's office during follow-upexaminations. During implant the system is used to record at least twomeasurements. The first measurement is taken during introduction of thesensor for calibration and the second measurement is taken afterplacement for functional verification. The measurements can be taken byplacing the coupling loop either on or adjacent to the patient's back orthe patient's stomach for a sensor that measures properties associatedwith an abdominal aneurysm. For other types of measurements, thecoupling loop may be placed in other locations. For example, to measureproperties associated with the heart, the coupling loop can be placed onthe patient's back or the patient's chest.

The system communicates with the implanted sensor to determine theresonant frequency of the sensor. As described in more detail in thepatent documents referenced in the Background section, a sensortypically includes an inductive-capacitive (“LC”) resonant circuithaving a variable capacitor. The distance between the plates of thevariable capacitor varies as the surrounding pressure varies. Thus, theresonant frequency of the circuit can be used to determine the pressure.

The system energizes the sensor with an RF burst. The energizing signalis a low duty cycle, gated burst of RF energy of a predeterminedfrequency or set of frequencies and a predetermined amplitude.Typically, the duty cycle of the energizing signal ranges from 0.1% to50%. In one embodiment, the system energizes the sensor with a 30-37 MHzfundamental signal at a pulse repetition rate of 100 kHz with a dutycycle of 20%. The energizing signal is coupled to the sensor via amagnetic loop. This signal induces a current in the sensor which hasmaximum amplitude at the resonant frequency of the sensor. During thistime, the sensor charges exponentially to a steady-state amplitude thatis proportional to the coupling efficiency, distance between the sensorand loop, and the RF power. FIG. 8 shows the charging response of atypical LC circuit to a burst of RF energy at its resonant frequency.The speed at which the sensor charges is directly related to the Q(quality factor) of the sensor. Therefore, the “on time” of the pulserepetition duty cycle is optimized for the Q of the sensor. The systemreceives the ring down response of the sensor via magnetic coupling anddetermines the resonant frequency of the sensor. FIG. 2(a) illustrates atypical energizing signal and FIGS. 2(b), 2(c) and 2(d) illustratetypical coupled signals for various values of Q (quality factor) for thesensor. When the main unit is coupling energy at or near the resonantfrequency of the sensor, the amplitude of the sensor return ismaximized, and the phase of the sensor return will be close to zerodegrees with respect to the energizing phase. The sensor return signalis processed via phase-locked-loops to steer the frequency and phase ofthe next energizing pulse. Operation of the Base Unit FIG. 3 is a blockdiagram of the signal processing components within an exemplary baseunit. The base unit determines the resonant frequency of the sensor byadjusting the energizing signal so that the frequency of the energizingsignal matches the resonant frequency of the sensor. In the embodimentillustrated by FIG. 3, two separate processors 302, 322 and two separatecoupling loops 340, 342 are shown. In one embodiment, processor 302 isassociated with the base unit and processor 322 is associated with acomputer connected to the base unit. In other embodiments, a singleprocessor is used that provides the same functions as the two separateprocessors. In other embodiments a single loop is used for bothenergizing and for coupling the sensor energy back to the receiver. Aswill be apparent to those skilled in the art, other configurations ofthe base unit are possible that use different components.

The embodiment illustrated by FIG. 3 includes a pair of phase lock loops(“PLL”). One of the PLLs is used to adjust the phase of the energizingsignal and is referred to herein as the fast PLL. The other PLL is usedto adjust the frequency of the energizing signal and is referred toherein as the slow PLL. The base unit provides two cycles: thecalibration cycle and the measurement cycle. In one embodiment, thefirst cycle is a 10 microsecond energizing period for calibration of thesystem, which is referred to herein as the calibration cycle, and thesecond cycle is a 10 microsecond energizing/coupling period forenergizing the sensor and coupling a return signal from the sensor,which is referred to herein as the measurement cycle. During thecalibration cycle, the system generates a calibration signal for systemand environmental phase calibration and during the measurement cycle thesystem both sends and listens for a return signal, i.e. the sensor ringdown. Alternatively, as those skilled in the art will appreciate, thecalibration cycle and the measurement cycle can be implemented in thesame pulse repetition period.

The phase of the energizing signal is adjusted during the calibrationcycle by the fast PLL and the frequency of the energizing signal isadjusted during the measurement cycle by the slow PLL. The followingdescription of the operation of the PLLs is presented sequentially forsimplicity. However, as those skilled in the art will appreciate, thePLLs actually operate simultaneously.

Initially the frequency of the energizing signal is set to a defaultvalue determined by the calibration parameters of the sensor. Eachsensor is associated with a number of calibration parameters, such asfrequency, offset, and slope. An operator of the system enters thesensor calibration parameters into the system via the user interface andthe system determines an initial frequency for the energizing signalbased on the particular sensor. Alternatively, the sensor calibrationinformation could be stored on portable storage devices, bar codes, orincorporated within a signal returned from the sensor. The initial phaseof the energizing signal is arbitrary.

The initial frequency and the initial phase are communicated from theprocessor 302 to the DDSs (direct digital synthesizers) 304, 306. Theoutput of DDS1 304 is set to the initial frequency and initial phase andthe output of DDS2 306 (also referred to as local oscillator 1) is setto the initial frequency plus the frequency of the local oscillator 2.The phase of DDS2 is a fixed constant. In one embodiment, the frequencyof local oscillator 2 is 4.725 MHz. The output of DDS 1 is gated by thefield programmable gate array (FPGA) 308 to create a pulsed transmitsignal having a pulse repetition frequency (“PRF”). The FPGA providesprecise gating so that the base unit can sample the receive signalduring specific intervals relative to the beginning or end of thecalibration cycle.

During the calibration cycle, the calibration signal which enters thereceiver 310 is processed through the receive section 311 and the IFsection 312, and is sampled. In one embodiment, the calibration signalis the portion of the energizing signal that leaks into the receiver(referred to herein as the energizing leakage signal). The signal issampled during the on time of the energizing signal by a sample and holdcircuit 314 to determine the phase difference between the signal andlocal oscillator 2. In the embodiment where the calibration signal isthe portion of the energizing signal that leaks into the receiver, thesignal is sampled approximately 100 ns after the beginning of theenergizing signal pulse. Since the energizing signal is several ordersof magnitude greater than the coupled signal, it is assumed that thephase information associated with the leaked signal is due to theenergizing signal and the phase delay is due to the circuit elements inthe coupling loop, circuit elements in the receiver, and environmentalconditions, such as proximity of reflecting objects.

The phase difference is sent to a loop filter 316. The loop filter isset for the dynamic response of the fast PLL. In one embodiment, the PLLbandwidth is 1000 Hz and the damping ratio is 0.7. A DC offset is addedto allow for positive and negative changes. The processor 302 reads itsanalog to digital converter (A/D) port to receive the phase differenceinformation and adjusts the phase sent to direct digital synthesizer 1(DDS 1) to drive the phase difference to zero. This process is repeatedalternatively until the phase difference is zero or another referencephase.

The phase adjustment made during the energizing period acts to zero thephase of the energizing signal with respect to local oscillator 2.Changes in the environment of the antenna or the receive chainimpedance, as well as the phase delay within the circuitry prior tosampling affect the phase difference reading and are accommodated by thephase adjustment.

During the measurement cycle, the energizing signal may be blocked fromthe receiver during the on time of the energizing signal. During the offtime of the energizing signal, the receiver is unblocked and the coupledsignal from the sensor (referred to herein as the coupled signal or thesensor signal) is received. The coupled signal is amplified and filteredthrough the receive section 311. The signal is down converted andadditional amplification and filtering takes place in the IF section312. In one embodiment, the signal is down converted to 4.725 MHz. Afterbeing processed through the IF section, the signal is mixed with localoscillator 2 and sampled by sample and hold circuits 315 to determinethe phase difference between the coupled signal and the energizingsignal. In one embodiment, the sampling occurs approximately 30 ns afterthe energizing signal is turned off.

In other embodiments, group delay or signal amplitude is used todetermine the resonant frequency of the sensor. The phase curve of asecond order system passes through zero at the resonant frequency. Sincethe group delay i.e. derivative of the phase curve reaches a maximum atthe resonant frequency, the group delay can be used to determine theresonant frequency. Alternatively, the amplitude of the sensor signalcan be used to determine the resonant frequency. The sensor acts like abandpass filter so that the sensor signal reaches a maximum at theresonant frequency.

The sampled signal is accumulated within a loop filter 320. The loopfilter is set for the dynamic response of the slow PLL to aid in theacquisition of a lock by the slow PLL. The PLLs are implemented withop-amp low pass filters that feed A/D inputs on microcontrollers, 302and 322, which in turn talk to the DDSs, 304 and 306, which provide theenergizing signal and local oscillator 1. The microcontroller thatcontrols the energizing DDS 304 also handles communication with thedisplay. The response of the slow PLL depends upon whether the loop islocked or not. If the loop is unlocked, then the bandwidth is increasedso that the loop will lock quickly. In one embodiment, the slow PLL hasa damping ratio of 0.7 and a bandwidth of 120 Hz when locked (theNyquist frequency of the blood pressure waveform), which isapproximately ten times slower than the fast PLL.

A DC offset is also added to the signal to allow both a positive and anegative swing. The output of the loop filter is input to an A/D inputof processor 322. The processor determines a new frequency and sends thenew frequency to the DSSs. The processor offsets the current frequencyvalue of the energizing signal by an amount that is proportional to theamount needed to drive the output of the slow PLL loop filter to apreset value. In one embodiment the preset value is 2.5V and zero inphase. The proportional amount is determined by the PLL's overalltransfer function.

The frequency of the energizing signal is deemed to match the resonantfrequency of the sensor when the slow PLL is locked. Once the resonantfrequency is determined, the physical parameter, such as pressure, iscalculated using the calibration parameters associated with the sensor,which results in a difference frequency that is proportional to themeasured pressure.

The operation of the slow PLL is qualified based on signal strength. Thebase unit includes signal strength detection circuitry. If the receivedsignal does not meet a predetermined signal strength threshold, then theslow PLL is not allowed to lock and the bandwidth and search window forthe PLL are expanded. Once the received signal meets the predeterminedsignal strength threshold, then the bandwidth and search window of theslow PLL is narrowed and the PLL can lock. In the preferred embodiment,phase detection and signal strength determination are provided via the“I” (in phase) and “Q” (quadrature) channels of a quadrature mixercircuit. The “I” channel is lowpass filtered and sampled to providesignal strength information to the processing circuitry. The “Q” channelis lowpass filtered and sampled to provide phase error information tothe slow PLL.

Avoiding False Locks

The system provides unique solutions to the false lock problem. A falselock occurs if the system locks on a frequency that does not correspondto the resonant frequency of the sensor. There are several types offalse locks. The first type of false lock arises due to the pulsednature of the system. Since the energizing signal is a pulsed signal, itincludes groups of frequencies. The frequency that corresponds to afalse lock is influenced by the pulse repetition frequency, the Q of thesensor, and the duty cycle of the RF burst. For example, a constantpulse repetition frequency adds spectral components to the return signalat harmonic intervals around the resonant frequency of the sensor, whichcan cause a false lock. In one embodiment, false locks occur atapproximately 600 kHz above and below the resonant frequency of thesensor. To determine a false lock, the characteristics of the signal areexamined. For example, pulse repetition frequency dithering and/orobserving the slope of the baseband signal are two possible ways ofdetermine a false lock. In one embodiment where the system locks on asideband frequency, the signal characteristics correspond to a heartbeator a blood pressure waveform.

The second type of false lock arises due to a reflection or resonance ofanother object in the vicinity of the system. This type of false lockcan be difficult to discern because it generally does not correspond toa heartbeat or blood pressure waveform. The lack of frequency modulationcan be used to discriminate against this type of false lock. Changingthe orientation of the magnetic loop also affects this type of falselock because the reflected false lock is sensitive to the angle ofincidence.

The third type of false lock arises due to switching transients causedby switching the PIN diodes and analog switches in the RF path. Thesetransients cause damped resonances in the filters in the receive chain,which can appear similar to the sensor signal. Typically, these types offalse locks do not correspond to a heartbeat or blood pressure waveformbecause they are constant frequency. These types of false locks are alsoinsensitive to orientation of the magnetic loop.

To avoid the first type of false lock, the present invention determinesthe slope of the baseband signal (the phase difference signal at point330). In one embodiment, if the slope is positive, then the lock isdeemed a true lock. However, if the slope is negative, then the lock isdeemed a false lock. In another embodiment, a negative slope is deemed atrue lock and a positive slope is deemed a false lock. The slope isdetermined by looking at points before and after the phase differencesignal goes to zero. The slope can be determined in a number ofdifferent ways, including but not limited to, using an analogdifferentiator or multiple sampling. FIGS. 4(a) and 4(b) illustrate atrue lock and a false lock respectively, when a positive slope indicatesa true lock. In one embodiment, if a false lock is detected, then thesignal strength is suppressed so that the signal strength appears to theprocessor 322 to be below the threshold and the system continues tosearch for the center frequency. In other embodiments, any non-zeroslope can be interpreted as a false lock resulting in zero signalstrength.

The system can also use frequency dithering to avoid the first type offalse lock. Since the spectral components associated with a constantpulse repetition frequency can cause a false lock, dithering the pulserepetition frequency helps avoid a false lock. By dithering the pulserepetition frequency, the spectral energy at the potential false lockfrequencies is reduced over the averaged sampling interval. As shown inFIG. 5, the energizing signal includes an on time t1 and an off time t2.The system can vary the on time or the off time to vary the PRF(PRF=1/(t1+t2)). FIG. 5 illustrates different on times (t1, t1′) anddifferent off times (t2, t2′). By varying the PRF, the sidebands moveback and forth and the average of the sidebands is reduced. Thus, thesystem locks on the center frequency rather than the sidebands. The PRFcan be varied between predetermined sequences of PRFs or can be variedrandomly.

Reducing Switching Transients

The coupling loop switches between an energizing mode and a couplingmode. This switching creates transient signals, which can cause thethird type of false lock. Phase dithering is one method used to reducethe switching transients. As shown in FIG. 6, the system receives aswitching transient 603 between the end of the energizing signal 602 andthe beginning of the coupled signal 604. To minimize the transient, thephase of the energizing signal may be randomly changed. However,changing the phase of the energizing signal requires that the systemredefine zero phase for the system. To redefine zero phase for thesystem, the phase of DDS2 is changed to match the change in phase of theenergizing signal. Thus, the phase of the energizing signal 602′ and thecoupled signal 604′ are changed, but the phase of the transient signal603′ is not. As the system changes phase, the average of the transientsignal is reduced.

Changing the resonant frequency of the antenna as it is switched fromenergizing mode to coupling mode also helps to eliminate the switchingtransients. Eliminating the switching transients is especially importantin the present invention because of the characteristics of the coupledsignal. The coupled signal appears very quickly after the on period ofthe energizing signal and dissipates very quickly. In one embodiment,the invention operates in a low power environment with a passive sensorso that the magnitude of the coupled signal is small. However, theinvention is not limited to working with a passive sensor.

The coupling loop is tuned to a resonant frequency that is based uponthe sensor parameters. Changing the capacitors or capacitor network thatis connected to the coupling loop changes the resonant frequency of theantenna. The resonant frequency typically is changed from approximately1/10% to 2% between energizing mode and coupled mode. In someembodiments, the coupling loop is untuned.

Additional alternative embodiments will be apparent to those skilled inthe art to which the present invention pertains without departing fromits spirit and scope. For example, the system can operate with differenttypes of sensors, such as non-linear sensors that transmit informationat frequencies other than the transmit frequency or sensors that usebackscatter modulations. Accordingly, the scope of the present inventionis described by the appended claims and is supported by the foregoingdescription.

1. A method for determining a resonant frequency of a wireless sensor,comprising: providing a calibration cycle, wherein the calibration cycleincludes: generating an energizing signal; receiving a calibrationsignal; and comparing the energizing signal and the calibration signalto determine a phase difference; and providing a measurement cycle,wherein the measurement cycle includes: energizing the wireless sensor;receiving a sensor signal from the wireless sensor; comparing the sensorsignal and a reference signal to determine a second phase difference;and using the second phase difference to determine the resonantfrequency of the wireless sensor.
 2. The method of claim 1, wherein thecalibration signal is an energizing leakage signal.
 3. The method ofclaim 1, wherein the reference signal is the energizing leakage signal.4. The method of claim 1, wherein the calibration cycle furthercomprises: adjusting a phase of the energizing signal until the phasedifference is a predetermined value.
 5. The method of claim 1, whereinthe measurement cycle further comprises: adjusting a frequency of theenergizing signal to reduce the second phase difference.
 6. The methodof claim 5, wherein using the second phase difference to determine thefrequency of the wireless sensor, comprises: using the frequency of theenergizing signal to determine the frequency of the wireless sensor. 7.The method of claim 5, wherein the measurement cycle is repeated untilthe second phase difference is a predetermined value.
 8. The method ofclaim 5, wherein the calibration cycle is repeated until the secondphase difference is a predetermined value.
 9. A method for determining aresonant frequency of a wireless sensor, comprising: providing acalibration cycle for determining a first relationship between anenergizing signal and a calibration signal; and providing a measurementcycle for receiving a sensor signal, determining a second relationshipbetween the sensor signal and the calibration signal and determining theresonant frequency of the wireless sensor based on the firstrelationship and the second relationship.
 10. The method of claim 9,wherein the first relationship is a phase difference.
 11. The method ofclaim 10, wherein the calibration cycle further comprises: adjusting aphase of the energizing signal until the phase difference is apredetermined value.
 12. The method of claim 9, wherein the firstrelationship is a time delay.
 13. The method of claim 9, wherein thesecond relationship is a phase difference.
 14. The method of claim 13,wherein the measurement cycle further comprises: adjusting a frequencyof the energizing signal until the second phase difference is apredetermined value.
 15. The method of claim 9, wherein the secondrelationship is a time delay.
 16. The method of claim 9, wherein a firstphase locked loop (PLL) is used to provide the calibration cycle and asecond PLL is used to provide the measurement cycle.
 17. The method ofclaim 16, wherein the first PLL is faster than the second PLL.
 18. Amethod for determining a resonant frequency of a wireless sensor,comprising: providing a calibration cycle, wherein the calibration cycleincludes: generating an energizing signal; receiving a calibrationsignal; and comparing the energizing signal and the calibration signalto determine a phase difference; and providing a measurement cycle,wherein the measurement cycle includes: energizing the wireless sensor;receiving a sensor signal from the wireless sensor; determining acharacteristic of the sensor signal; and using the characteristic todetermine the resonant frequency of the wireless sensor.
 19. The methodof claim 18, wherein the characteristic is amplitude and wherein amaximum amplitude of the sensor signal is used to determine the resonantfrequency of the sensor.
 20. The method of claim 18, wherein thecharacteristic is sensor group delay and wherein a maximum sensor groupdelay of the sensor signal is used to determine the resonant frequencyof the sensor.